The present invention relates to a method for continuous displacement washing of liquid-containing pulp materials, wherein the pulp is deposited on a moving filter, displacement liquid is supplied to the pulp and a difference in pressure is maintained across the pulp to replace the liquid therein with displacement liquid and recover substantially undiluted liquid from the pulp.
The method according to the invention may be used on various pulps such as fibrous pulp in the wood-pulp industry, pulps of granular particles such as mash in breweries, pulps of comminuted crushed sugar cane, etc. The invention is not dependent upon the formation of a pulp cake such as in the washing of pulps in the wood-pulp industry, but the pulp must be deposited in such a way that a reasonably uniform pulp distribution is obtained. The invention is especially developed for recovering liquor from sulphate pulp and will be described below on the basis of this use.
For recovering black liquor from sulphate pulp there are used batteries of diffusers, continuous diffusers in which the displacement liquid must traverse a distance of approximately 30 cm through the pulp, and continuous filters on which the pulp is deposited in mats of 2 cm or more. Washing liquid is supplied to the surface of the pulp and shall penetrate the pulp at a rate that should be as uniform as possible over the entire surface of the pulp in order that the liquid initially emerging from the opposite side of the pulp through the filter shall be as undiluted as possible. However, it is difficult to deposit the pulp on the filter in such a manner that the pulp will not afford a locally reduced flow resistance. In places having a reduced flow resistance the washing liquid will flow much faster through the pulp than in the remaining places, resulting in an undesired dilution of the black liquor recovered from the pulp. No layer of liquid is maintained over the pulp, probably because the washing liquid is not to flow laterally to places where the flow resistance is least, but is to act through the entire pulp. Instead, nozzles spaced as uniformly as possible over the surface of the pulp are used, which nozzles sprinkle water onto the surface. However, such a sprinkling through nozzles can never provide an entirely uniform water supply. The supply will always be greater locally. Also this fact leads to a dilution of the displaced liquid.